Metal-air batteries use a negative electrode based on a metal such as zinc, iron or lithium, coupled to an air electrode. The electrolyte used most often is an alkaline aqueous electrolyte.
During discharging of such a battery, oxygen is reduced at the positive electrode and the metal is oxidized at the negative electrode:                Discharge at the negative electrode: M→n++n e−        Discharge at the positive electrode: O2+2H2O+4e−→4 OH−        
When a metal-air battery has to be recharged electrically, the direction of the current is reversed. Oxygen is produced at the positive electrode and the metal is redeposited by reduction at the negative electrode:                Recharge at the negative electrode: Mn++n e−→M        Recharge at the positive electrode: 4OH−→O2+2H2O+4e−        
Metal-air systems have the advantage of using a positive electrode of infinite capacity. Electrochemical generators of the metal-air type are therefore known for their high specific energies, which can reach several hundred Wh/kg. The oxygen consumed at the positive electrode does not need to be stored in the electrode but can be taken from the surrounding air. Air electrodes are also used in alkaline fuel cells, which are particularly advantageous compared with other systems owing to the high reaction kinetics at the level of the electrodes and owing to the absence of noble metals such as platinum.
Problems during the recharging of batteries of the metal-air type have yet to be solved. In particular, the air electrode, which is the positive electrode of the battery during discharging, is not designed to be used in the recharging direction.
An air electrode is a porous solid structure in contact with the liquid electrolyte. The interface between the air electrode and the liquid electrolyte is a so-called “triple-contact” interface, where the active solid material of the electrode, the gaseous oxidant, i.e. air, and the liquid electrolyte are present simultaneously. A description of the different types of air electrodes for zinc-air batteries is presented for example in the bibliographic article by V. Neburchilov et al., with the title “A review on air cathodes for zinc-air fuel cells”, Journal of Power Sources 195 (2010) pp. 1271-1291.
The air electrode is usually composed of carbon particles with a large surface area such as Vulcan® XC72 marketed by Cabot. The surface area of the carbon can be increased by reaction with a gas, such as CO, prior to its integration in the air electrode. A porous electrode is then produced by agglomeration of the carbon particles using a fluorinated hydrophobic polymer such as FEP (fluorinated ethylene propylene) marketed by the company DuPont. Patent WO 2000/036677 describes such an electrode for a metal-air battery.
It is preferable to have a reaction surface area on the air electrode that is as large as possible, in order to have a current density relative to the geometric surface area of the electrode that is as high as possible. A large reaction surface area is also useful because the density of gaseous oxygen is low compared with a liquid. The large surface area of the electrode allows the reaction sites to be multiplied. Conversely, this large reaction surface area is no longer necessary for the reverse reaction of oxidation during recharging since the concentration of active material is much higher.
The use of an air electrode during charging to bring about an oxidation reaction and evolution of oxygen presents many drawbacks. The porous structure of the air electrode is fragile. It was observed by the inventors that this structure was destroyed mechanically by the evolution of gas when it was used to produce oxygen by oxidation of a liquid electrolyte. The hydraulic pressure generated within the electrode by the production of gas is sufficient to cause the bonds between the carbon particles constituting the air electrode to rupture.
It was also observed by the inventors that the catalyst added to the air electrode to improve the energy yield of the reaction of reduction of oxygen, such as manganese oxide or cobalt oxide, is not stable at the potential required for the reverse oxidation reaction. The corrosion of carbon in the presence of oxygen by oxidation of carbon is also accelerated at higher potentials.
Some use a more resistant oxygen reduction catalyst coupled to an oxygen evolution catalyst in a bifunctional electrode composed of two electrically coupled layers, as described in patent U.S. Pat. No. 5,306,579. However, this configuration produces electrodes that nevertheless have a short service life and a limited number of cycles.
The degradation of the air electrode, when it is used to recharge the metal-air battery, greatly reduces the battery's service life. This is one of the main reasons for the low level of commercial development of electrically rechargeable metal-air accumulators.
Faced with these problems, one of the means that has been adopted for protecting the air electrode against degradation consists of using a second positive electrode, which is used for the oxygen evolution reaction. The air electrode is then decoupled from the oxygen evolution electrode and only the latter is used during the charging phase. For example, patent U.S. Pat. No. 3,532,548 of Z. Starchurski describes a zinc-air battery with a second auxiliary electrode used for the charging phase. During the charging phase, the air electrode is therefore inactive. As far as the inventors know, it has never been suggested that this air electrode could be used for anything during the step of charging the battery.
Moreover, as with all batteries, it is important to monitor and control the voltage at the terminals of a battery of the metal-air type while it is being discharged and recharged. The voltage at the terminals of a battery is generally measured without difficulty directly between the negative terminal and the positive terminal of the battery. The voltage at the terminals of the battery represents the potential difference of the positive electrode and negative electrode.
In the case of conventional batteries, the voltage can be controlled by electronic control systems or a BMS (battery management system). These devices are well known to a person skilled in the art. The aim of a BMS is to monitor the state of the various elements of the battery, as well as to protect it from the degradation that could be caused by improper use, for example overvoltage or undervoltage. The BMS therefore also has the function of increasing the battery's service life.
It was found by the inventors that simply controlling the voltage at the terminals of batteries of the metal-air type could be insufficient for optimum protection of the battery against certain types of degradation occurring on the negative electrode side during electrical recharging of a metal-air battery.
For example, in a zinc-air battery, during recharging, the Zn2 metal ions are reduced at the negative electrode and are deposited in their metallic form Zn once the potential at the level of this electrode is sufficiently negative. A uniform and homogeneous deposit of metal on the electrode is desired for ensuring good durability during the cycles of charging and discharging of this battery.
It was found that, under certain conditions, the metal was deposited in the form of foam with little adherence to the surface of the electrode, and this foam could then become detached from the electrode, causing a loss of active material and consequently a loss of specific capacity of the battery. In other cases, it was found that the metal could also be deposited in the form of dendrites. These dendrites can grow until they reach the positive electrode during charging, causing an internal short-circuit, preventing recharging.
It was observed by the inventors that controlling the potential of the negative electrode during charging to prevent it becoming too high makes it possible to limit the formation of zinc deposits in the form of foam or dendrites.
However, in the case of metal-air batteries, it is known that, during charging, the potential of the positive electrode increases much more quickly than the potential of the negative electrode. Because of this, the control of the voltage at the terminals of the battery is not sufficiently precise to provide control of the potential of the negative electrode.
Therefore there is at present a need for an accurate means of measuring and controlling the potential of the negative electrode of a battery of the metal-air type while it is being charged.